Question

I am pretty new to iPhone programming, and was playing around with an app yesterday trying different scenarios with view controllers and nib files. So, I started a new app with a FirstViewController (FVC for short) and an FVC.xib.

I layed out a quick view in FVC.xib and ran the app – view displays, great.

I now wanted to have a second view I could add on top of the main view. So I went ahead and created SecondViewController.xib (SVC) but did not create the .m and .h files. I went about trying to load both these views from the same view controller, and here is where my question lies:

I created a button in FVC.xib and created an IBAction like this:

- (IBAction)loadSVC {
    FirstViewController *viewController = [[FirstViewController alloc] initWithNibName:@"SecondViewController" bundle:[NSBundle mainBundle]];
    secondView = viewcontroller.view;
    [viewController release];
    [self.view addSubView:secondView];
}

So this works great and adds the contents of SVC.xib, but when I try and remove that view from the superview, the app crashes:

[secondView removeFromSuperview];

If I actually create a view controller for SVC, use that to instantiate my view in FVC, and move the remove code to the SVC:

[self.view removeFromSuperview];

Everything works. My question – I kind of get why my first method crashes, but I was hoping someone could explain why and what goes on behind the scenes. I’m still a noob with object oriented programming, so what is actually happening in my first case where I create a new instance of FirstViewController and add its view to self.view? Why can’t I release it (I assume because the original view is associated with FirstViewController, and when I create a new instance with the second xib it messes everything up) – I’d love a more technical explanation as to what is happening…

Thanks much!!

EDIT to add more info in response to Nick’s reply below

Nick – so your answer did clear my thinking a bit in regards to the retain count, etc… I did another test app trying to get this working from a single view controller – think, for example, that I wanted to display an Alert or Welcome message to the user (I know in a real app there are different methods to accomplish this, but this is more of a learning experience) — so I have my main view @ MainViewController and layout my alert message in a xib called alert.xib — so there is no logic behind the alert message, no reason for it to have a view controller that I can see, my end goal being loading/unloading this on top of my main view from the main view’s view controller (or understanding why it is impossible)

I tried this using instance variables as you recommended:

In MainViewController.h:

#import <UIKit/UIKit.h>

UIViewController *secondController;
UIView *secondView;

@interface MainViewController : UIViewController {

}

@property(nonatomic, retain) UIViewController *secondController;
@property(nonatomic, retain) UIView *secondView;

- (IBAction)loadSecond;
- (IBAction)removeSecond;

@end

In MainViewController.m:

#import "MainViewController.h"

@implementation MainViewController

@synthesize secondController, secondView;

- (IBAction)loadSecond {
secondController = [[MainViewController alloc] initWithNibName:@"alert" bundle:[NSBundle mainBundle]];
secondView = secondController.view;
[self.view addSubview:secondView];
}

- (IBAction)removeSecond {
//I've tried a number of things here, like [secondView removeFromSuperview];, [self.secondView removeFromSuperview];, [secondController.view removeFromSuperview];
}
- (void)dealloc {
[secondController release];
[secondView release];
[super dealloc];
}

So – this works to load the alert view, but the removeSecond button does nothing (I did use NSLog to verify the removeSecond method is fired) – why?

Second, and most importantly – is this even possible, or is it horrible practice? Should every nib/view I am manipulating have their own view controller? Am I wrong to think I could just make a new instance of MainViewController and use it to display and remove this no-functionality, very temporary view? (And yes, I realize I could easily create this view programatically or accomplish the end goal in many different ways which would be easier, but I’m trying to really learn this stuff and I think figuring this out will help…

Thanks for the help!

Answer 1

1. Based on your clarifications, you don’t need secondView property or iVar. Also in your loadSecond instead of secontController = bla you need self.secondController = bla, otherwise you simply assign reference to the iVar instead of going through the setter.
2. Yes, it’s possible to load subviews/other resources from a nib without having a dedicated controller

This is how you do it (one of the approaches):

UIView *result = nil;
NSArray *bundle = [[NSBundle mainBundle] loadNibNamed:@"MyNibName" owner:owner options:nil];
for ( id o in bundle ) {
    if ( [o isKindOfClass:[UIView class]] ) {
        result = (UIView *)o;
        break;
    }
}

Here the result will contain the first UIView in MyNibName. You can use other criteria to find out whether you got the view you wanted (tags, types…)